1. Field of the Invention
The present invention relates to a digital broadcasting apparatus, and more particularly, to a phase-compensation decision feedback equalizer for removing a frequency offset and a phase jitter of a carrier wave and demodulating a passband digital signal into a baseband digital signal.
2. Discussion of the Related Art
A digital broadcasting reception technology has been developed in several forms of media (ground wave, cable, satellite). Meanwhile, the digital broadcasting reception technology is now developing in a form of an integration system for the integrated digital broadcasting.
Specifically, since a transmission system of an HDTV transmits a data having a high bit rate (more than 20 Mb/s) through a band-limit of 6 MHz, a modulation/demodulation method having good bandwidth efficiency is required. Also, a symbol/carrier recovery method for recovering a symbol and a carrier wave and a channel equalizer for compensating for abnormal channel characteristics such as a multi-path and a Doppler effect are required. A channel demodulation method having a good efficiency is used to reliably transmit data with respect to noise.
An integrated “multi-channel digital broadcasting receiver” is largely classified into three receiving system.
1) Ground wave broadcasting receiving system:                VSB (vestigial sideband, SSB, ATSC, American method)        OFDM (Orthogonal Frequency Division Multiplexing, DSB, European method)        
2) Cable broadcasting receiving system                QAM (Quadrature Amplitude Modulation, DSB)        
3) QPSK (Quadrature Phase Shift Keying, DSB)
The three receiving systems are implemented with different technologies, such as an analog reception, a signal synchronization, a channel equalization, a matched filter and a channel demodulation.
However, there is a demand for development of a “multimedia digital broadcasting receiver” that is optimized by selecting sharable functions among those technologies.
The multimedia digital broadcasting receiver will be described with reference to FIG. 1.
FIG. 1 is a block diagram of a general multimedia digital broadcasting receiver. Referring to FIG. 1, the multimedia digital broadcasting receiver includes an analog receiving part 101 for converting a radio frequency (RF) signal into an intermediate frequency (IF) signal, and a digital demodulating part 102 for demodulating a digital signal that is converted by an analog-to-digital converter.
In the analog receiving part 101, an RF signal of 50-860 MHz is received through a tuner 101b and converted into an IF signal. The converted signal is converted into a digital signal by the analog-to-digital converter 101h and transmitted to the digital demodulating part 102.
The analog receiving part 101 includes: a tuner 101 for receiving an RF signal of 50-860 MHz through an antenna 101a and converting it into a first IF signal of 44 MHz; a surface acoustic wave (SAW) filter 101c for filtering an output signal of the tuner 101b; a first oscillator 101d for generating an oscillation frequency used to generate a second IF signal; a mixer 101e for converting the filtered signal into a second IF signal by down-converting the filtered signal to the oscillation frequency of the first oscillator 101d; an automatic gain control (AGC) amplifier 101f for compensating for a gain of the second IF signal; a second oscillator 101g for generating a sampling frequency; and an analog-to-digital converter 101h for converting the amplified signal into a digital signal, based on the sampling frequency generated from the second oscillator 101g. 
At this point, the analog receiving part 101 has a fixed structure shown in FIG. 1, regardless of the demodulation methods.
On the other hand, the digital demodulating part 102 has various structures depending on the modulation methods (VSB, OFDM, QAM, QPSK, etc.). A symbol recovery, a matched filter, a phase splitter, a carrier recovery, a channel equalizer and an FEC are used as basic elements.
The symbol recovery is fed back with a timing error of current symbols by using a resampler, and interpolates the digital signal outputted from the analog-to-digital converter 101h so as to reduce a signal error and then transmits it to the matched filter.
In the symbol recovery, a process of converting an analog data into a digital data by using a fixed frequency and a process of recovering a clock of all symbol sequence are preformed in a digital domain. Therefore, other analog devices except the analog-to-digital converter are not needed such that the symbol recovery can be simplified and a device noise can be removed.
The matched filter filters a signal outputted in synchronization with a symbol at the symbol recovery and re-adjusts a signal to noise ratio (SNR) to the maximum. A reception filter theoretically has a rectangular bandwidth characteristic and cannot implement a shape of a reception filter having an infinite time delay. Accordingly, the detection process at a system having such a characteristic is sensitive to a very slight timing error. Although an inter-symbol influence at an accurate sampling time can be avoided, an inter-symbol interference occurs when a slight error exists. Thus, an excess bandwidth is required in order to implement the system. For this purpose, the matched filter is used.
The phase splitter separates the output of the matched filter into a real part and an imaginary part, that is, I signal and Q signal, and then transmits them to the carrier recovery part.
The carrier recovery part removes a frequency offset and phase jitter caused by the tuner 101a and the mixer 101d. Also, the carrier recovery part demodulates the passband digital signal into the baseband digital signal.
The channel equalizer removes an inter-symbol interference.
That is, in the digital transmission system such as an HDTV, distortion occurring while the transmission signal passes through the multi-path channel, interference due to an NTSC, or distortion in the transceiver system causes a bit detection error in a receiver side. Specifically, since signal propagation through the multi-path causes an inter-symbol interference (ISI), it is a main cause of the bit detection error. In order to solve it, the channel equalizer is used.
The FEC uses an RS encoding and a grid modulation encoding to remove a burst noise and sporadic noise, which exist on a channel contained in a signal whose inter-symbol interference is removed. Then, the FEC recovers a synchronizing signal inserted in the transmission from the baseband signal, and the received data (that is, the transmission symbol) is recovered using the synchronizing signal.
The phase-compensation decision feedback channel equalizer will be described below in detail.
The phase-compensation decision feedback channel equalizer used in the transmission system of the HDTV removes a ghost occurring while the transmission signal passes through the multi-path channel, interference due to an NTSC, or distortion in the transceiver system, which causes a bit detection error in a receiver side. Specifically, the phase-compensation decision feedback channel equalizer compensates for a main cause of the bit detection error, which generates an inter-symbol interference (ISI) of the transmission signal.
FIG. 2 is a block diagram of a digital broadcasting receiver having a phase-compensation decision feedback channel equalizer. The digital broadcasting receiver includes a pre-processing unit 100, a phase-compensation decision feedback channel equalizer 200, and a post-processing unit 300, which are disposed in this order.
The pre-processing unit 100 converts a passband digital signal into a baseband digital signal and outputs the baseband digital signal to the phase-compensation decision feedback channel equalizer 200.
The phase-compensation decision feedback channel equalizer 200 compensates for a ghost occurring while the baseband digital signal passes through the channel, interference due to an NTSC, or distortion in a transceiver system, which causes a bit detection error in a receiver side. Specifically, the phase-compensation decision feedback channel equalizer 200 compensates for a main cause of the bit detection error, which generates an inter-symbol interference (ISI) in the signal propagation through the multi-path. Then, the phase-compensation decision feedback channel equalizer 200 outputs the baseband digital signal, whose noise is removed, to the post-processing unit 300. The post-processing unit 300 removes a noise existing on the channel and recovers the transmission symbol.
FIG. 3 is a block diagram of a conventional phase-compensation decision feedback channel equalizer 200. The conventional phase-compensation decision feedback channel equalizer includes a feedforward filter 201, a feedback filter 202, an adder 203, and a decider/error generator 204.
An output of the decision feedback channel equalizer is a baseband signal where a phase of a carrier wave and a phase of a ghost exist together, and a constellation of a signal inputted to the feedforward filter 201 is inclined by a residual phase and transmitted to an input of the feedforward filter 201. Meanwhile, then input of the feedback filter is a decision signal constellation produced by slicing the output of the phase compensator 210. At this point, the constellation of the signal inputted to the feedback filter 202 is compensated by the phase compensator and thus shows an upright constellation without any distortion.
The adder adds the output signal having the inclined phase to the output signal having the upright phase. An output of the adder causes the distortion of the constellation due to different phases, and thus the signal having the distorted constellation is outputted.
Consequently, the constellation degraded due to the residual phase shows a constellation similar to the residual phase of the phase-compensation decision feedback channel equalizer. The residual phase is compensated by the phase compensator, such that it has an upright constellation.
In the digital broadcasting receiver, an input constellation of the feedback filter 202 in the decision feedback channel equalizer generates a phase error as much as the output constellation of the feedforward filter 201 and the residual phase. Since this phase error is added to the two constellations, the performance in the final channel compensation of the channel equalizer varies with the phase of the carrier wave and the phase of the ghost. Consequently, the entire ghost removal performance is degraded.